The present invention relates to eliminating a current sensing resistor across the base to emitter junction of a bipolar transistor so as to limit circuits for removal of a gate drive signal when the drain current is too high while preventing the current from going any higher.
Today, integral current-limiting circuits are widely used with power MOSFET devices. One of the primary reasons for incorporating the current-limiting circuits is to provide protection for the power MOSFET devices in the event of a shorted load. The shorted load might otherwise allow the current to rise high enough to destroy the power MOSFET devices before fault sensing circuitry can detect and interrupt the fault.
FIG. 1 illustrates a MOSFET device 10 having a metal-oxide semiconductor (MOS) and a bipolar transistor, that is commonly used in power MOSFET devices, formed on a silicon substrate. A current flow through the MOS is sensed by a current-sensing resistor 12 across a base-to-emitter junction of the bipolar transistor. The bipolar transistor having a collector 18, an emitter 16, and a base 14 and the MOS having a gate 15, a drain 17, and a source 19. The collector 18 is tied to the gate 15 of the MOS that includes a terminal 20 formed on a gate metal 24. The emitter 16 is tied to the source 19 of the MOS that includes a source terminal 22 formed on a source metal 26. The bipolar transistor of the power MOSFET device 10 further includes a base 14.
When the voltage drop on the sense-resistor 12 reaches a voltage value VBE of the bipolar transistor of about 0.7 to 0.8 volt, the bipolar transistor becomes conductive and removes enough of the drive signal from the MOS gate 20 to prevent the current from rising any higher. One of the disadvantages of the power MOSFET device 10 is in that the resistance of the sense resistor 12 must be high enough to produce about 0.7 to 0.8 volt at the current-limiting level. And this resistance of the sense resistor 12 is directly in series with the MOSFET ON-resistance, resulting in increased power dissipation during conduction.
Many applications exist, which, as shown in FIG. 2, connect the two described devices, a bipolar transistor 32 and a MOS 34, back-to-back, forming a bi-directional current-limiting switch 30 to control an alternating current. FIG. 2 only shows one device. A disadvantage of such circuit 30 is in that the load current must always flow through two in series sense resistors 36, further increasing the power dissipation.
What is needed is a way to remove some of the gate drive signal when the drain current is too high, so as to prevent the current from going any higher, while eliminating use of a current sensing resistor across the base to emitter junction of a bipolar transistor required in available current limiting circuits.